The present invention relates generally to AC induction motors and, more particularly, to apparatus for determining the rotational speed of an AC induction motor based on a spectral analysis of a magnetic flux signal.
When applying automated techniques involving spectral analysis of magnetic flux or motor current signals for recognizing induction motor faults, such as broken rotor bars, and for proactively predicting failures due to overheating in time for preventative maintenance action, it is usually necessary to determine, to a high degree of accuracy, the actual rotational speed of a motor. The various analysis approaches require information regarding motor speed, and yet the speed of an AC induction motor varies with load.
Examples of automated fault detection systems which implement spectral analysis for determining rotational speed are disclosed in Kliman et al U.S. Pat. No. 4,761,703 and Kliman U.S. Pat. No. 5,049,815. A system which advantageously can employ the subject invention is disclosed in concurrently-filed Bowers et al application Ser. No. 08/320,152, filed Oct. 7, 1994, entitled "Proactive Motor Monitoring for Avoiding Premature Failures and for Fault Recognition". Analysis of flux signals is noninvasive, relatively fast, and can be accomplished without taking the motor out of service.
An electric motor by definition produces magnetic flux. Any small imbalance in the magnetic or electric circuit of the motor effectively magnifies axially transmitted fluxes, and a flux coil may be employed as a sensor for detecting flux signals. A frequency spectrum analysis, such as by employing a Fast Fourier Transform (FFT), reveals the existence of a great many frequency components, with relatively complex relationships. In most cases, the frequency spectrum in principle contains sufficient information to determine motor speed.
Kliman et al U.S. Pat. No. 4,761,703 among other things discloses an analysis technique whereby analysis of a flux frequency spectrum is employed to determine slip frequency and thus running speed, since slip is the difference between motor synchronous speed and the actual running speed. (The synchronous speed of an AC induction motor is determined by the formula two times the AC power line frequency divided by the number of motor poles. An AC induction motor which is unloaded runs nearly at its synchronous speed, and runs at lower speeds as load increases.)
More particularly, Kliman et al, in U.S. Pat. No. 4,761,703, disclose an approach whereby an FFT is performed on time domain data acquired by sampling a flux signal, and the maximum FFT bin of the resulting flux spectra in a predefined frequency range, typically between 0.1 Hz and 1.5 Hz is found. A "best estimate" of the slip frequency is then computed using an interpolation technique described by Kliman et al. This approach can fail if no peaks are present in the 0.1 Hz to 1.5 Hz frequency range as can occur for certain speed motors. Also, there sometimes is excessive noise in signals at such low frequencies.
Kliman, in U.S. Pat. No. 5,049,815, discloses a different technique. Rather than analyzing a flux signal, in the disclosure of U.S. Pat. No. 5,049,815 it is proposed to analyze a motor current signal. In any event, as disclosed in Kliman U.S. Pat. No. 5,049,815, sideband peaks about line frequency are analyzed, looking for symmetrical pairs of sidebands as indicators of slip frequency. The slip frequency candidates are then subjected to further qualifying tests as described in the Kliman patent.
While the approach disclosed in the Kliman et al and Kliman patents may very well be effective for the particular motors analyzed, there remains a need for a general purpose approach to determining motor rotational speed, based particularly on magnetic flux signals, which works reliably with a wide variety of AC induction motors of different characteristics.
Determining motor rotational speed from magnetic flux signals or even current signals is not particularly simple or straightforward. The disclosure of Kliman U.S. Pat. No. 5,049,815 illustrates this complexity to some extent. This is particularly so in the general case, when the characteristics of a given motor are not well known in advance. For example the characteristics of a 12-pole 60 Hz AC induction motor with a synchronous speed of 600 RPM are very different from the characteristics of a 2-pole 60 Hz AC induction motor with a synchronous speed of 3600 RPM. Thus, in a general purpose instrument employed to maintain a variety of AC induction motors and associated equipment in a facility, it is a requirement that accurate results be obtained with a wide variety of motors of various sizes and configurations.